The hydrochlorofluorocarbon 141b is a replacement for trichlorofluoromethane ("11") as a foam blowing agent. As shown by Brooks et al. in U.S. Pat. No. 4,948,479 (the entire specification of which is hereby incorporated by reference), however, the manufacture of 141b from 140a and hydrogen fluoride ("HP") results in the formation of vinylidene chloride ("1130a") and other unsaturated by-products and acids, principally HCl. Because of similar boiling points, 141b (b.p. 32C) and 1130a (b.p. 31C) cannot be readily separated by distillation. Thus, Brooks et al. teach the use of a photochlorination step to convert the 1130a in the 141b product stream to the higher boiling 130a (b.p. 130.5C), then distillation of the photochlorinated liquid mixture to separate the low boiling 141b from the higher boiling liquids such as 130a (and possibly 140a unless it has been removed upstream). However, it is found that 1130a and acid (primarily HCl with lesser amounds of HF) are reformed in the distillation column and distill overhead with the 141b product. While the acid can be removed from the product by selective adsorption or reaction, such as by passing the acidic 141b through a bed of potassium hydroxide, it is undesirable since it is an added step. Reformation of the 1130a is more difficult, however, because as noted above it has nearly the same boiling point as the desired 141b product. Thus, yet another post-treatment would be required to remove the 1130a unless its formation in the distillation column can be inhibited. The same problem is presented when 141b is formed by the reaction of 1130a and HF [as taught, for example, by Henne et al., JACS 65, 1271 (1943)], resulting in a product stream containing 141b and 1130a.